ABB Relion 620 Series - Page 666

4.5.6.5 Application
A continuous or temporary voltage unbalance can appear in the network for
various reasons. The voltage unbalance mainly occurs due to broken conductors or
asymmetrical loads and is characterized by the appearance of a negative-sequence
component of the voltage. In rotating machines, the voltage unbalance results in a
current unbalance, which heats the rotors of the machines. The rotating machines,
therefore, do not tolerate a continuous negative-sequence voltage higher than
typically 1-2 percent x U
n
.
The negative-sequence component current I
2
, drawn by an asynchronous or a
synchronous machine, is linearly proportional to the negative-sequence component
voltage U
2
. When U
2
is P% of U
n
, I
2
is typically about 5 x P% x I
n
.
The negative-sequence overcurrent NSPTOC blocks are used to accomplish a
selective protection against the voltage and current unbalance for each machine
separately. Alternatively, the protection can be implemented with the NSPTOV
function, monitoring the voltage unbalance of the busbar.
If the machines have an unbalance protection of their own, the NSPTOV operation
can be applied as a backup protection or it can be used as an alarm. The latter can
be applied when it is not required to trip loads tolerating voltage unbalance better
than the rotating machines.
If there is a considerable degree of voltage unbalance in the network, the
rotating machines should not be connected to the network at all. This logic can
be implemented by inhibiting the closure of the circuit breaker if the NSPTOV
operation has started. This scheme also prevents connecting the machine to the
network if the phase sequence of the network is not correct.
An appropriate value for the setting parameter
Voltage start value is approximately
3 percent of U
n
. A suitable value for the setting parameter Operate delay time
depends on the application. If the NSPTOV operation is used as backup protection,
the operate time should be set in accordance with the operate time of NSPTOC
used as main protection. If the NSPTOV operation is used as main protection, the
operate time should be approximately one second.
4.5.6.6 Signals
Table 621: NSPTOV Input signals
Name Type Default Description
U
2
SIGNAL 0 Negative phase se-
quence voltage
BLOCK BOOLEAN 0=False Block signal for acti-
vating the blocking
mode
Table 622: NSPTOV Output signals
Name Type Description
OPERATE BOOLEAN Operate
START BOOLEAN Start
Protection functions 1MRS757644 H
666 620 series
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